The field of the disclosure relates generally to actuators and motors and, more particularly, to power control systems for linear switched capacitance actuators and motors.
Many known motors/actuator devices use magnetic fields as a force transfer mechanism rather than electric fields due to the higher energy densities achieved with magnetic fields using conventional materials and configurations. Such known devices are sometimes referred to as electromechanical actuators (EMAs). At least some of these EMAs include at least one electric motor as a driving device, such motor coupled to one of an alternating current (AC) power source and/or a direct current (DC) power source. Some of these known motor-driven EMAs may also include a power transfer device, e.g., a geared transmission or a direct drive shaft. However, such known EMAs have some disadvantages for smaller applications, such as operation of robot translatables and aviation devices.
At least some other known motors and actuators use electric fields rather than magnetic fields for electro-mechanical energy transfer. A switched capacitance actuator (SCA) is an electric field-based device that demonstrates an improved energy density over earlier electric field-based devices. The electro-mechanical energy conversion is at least partially a result of the change in the device capacitance with respect to rotor translation. Such SCAs are electrostatic motors that include a translatable portion, e.g., a rotor and a stationary portion, e.g., a stator and operate in a manner similar to the magnetic field equivalent of the SCA, i.e., a switched reluctance motor (SRM).
However, such known SCAs do not match electromagnetic machines with respect to the motion inducing shear stress, i.e., total force or torque output per unit rotor surface area. Typically, magnetically coupled actuators have gravimetric power densities below 1 kiloWatt per kilogram (kW/kg). In comparison, typical hydraulic actuators have gravimetric power densities on the order of 3-5 kW/kg, however, such typical hydraulic actuators have low efficiencies. Therefore, to attempt to achieve parity with electromagnetic devices with respect to power-to-weight ratio, at least some known SCAs compensate for the relatively lower shear stress by increasing the active area of the air gap defined by the SCA rotor and stator. According to Gauss' theorem, electric field lines are not required to define closed field loops, and in contrast, magnetic field lines form closed loops that originate and terminate on the magnet. Since the electric field lines do not need to be closed, the rotor surface area may be increased by adding active layers.
In order to maintain constant force, magnetically coupled devices, such as EMAs, maintain constant current, whereas electrostatic devices, such as known SCAs maintain constant voltage. As such, SCAs usually require a high voltage and a low current for operation, whereas EMAs are operated at comparatively higher current and lower voltages. Due to a lower shear stress, SCAs tend to require more surface area than EMAs. This requirement, coupled with the fact that electric field lines terminate on charges, leads to a machine structure for many known SCAs which resembles a stack of interleaved stationary and translatable plates, i.e., circuit boards including a plurality of unit cells.
Such known SCAs have limited power regulation features. For example, such known SCAs have limited regulatory control of the total actuator force or torque induced. One simple implementation of power regulation is to apply the same voltage to the entire actuator, i.e., each board and unit cell are exposed to the same poly-phase excitation. However, if the commanded force/torque is low compared to the rated machine force/torque, poor dynamic performance may result, discretization electrical chatter may become prominent, or the SCA may operate in a low efficiency manner. In addition to low force induction and noisy and inefficient operation, the electrical structure used to induce a singular voltage to all circuit boards reduces the fault tolerance of the SCA.